The invention relates generally to a method to enhance atomization of liquid fuel for combustion. In particular, the atomization is increased by pre-mixing an atomizing gas into the liquid fuel before it is ejected into the combustion chamber.
The design of this invention may be used to the advantage in internal combustion engines, and especially jet engines, as well as any other reciprocating engine, furnace, turbine, or combustor of liquid fuel where the atomization of liquid fuel is desired before its combustion.
Modern day aircraft engines also require efficient fuel burning inside the combustion chamber to convert this energy into thrust and produce less toxic emissions. Typically these engines use some sort of high pressurized fuel injectors to spray fuel into the combustion air. High injection pressures and high momentum air flows are required to generate small fuel drops and homogenous mixtures. Thus the fuel, after breaking up and mixing with atomizing gas, forms a fine spray pattern as it is discharged into the combustion chamber.
As more and more stringent rules are imposed on emissions regulations, efficient atomization of fuel is necessary to produce combustion with required levels of emissions. One key measurable parameter for spray quality is the Sauter Mean Diameter (SMD) of the spray droplets, with respect to which the smaller the SMD, the better the spray quality. Unfortunately, to achieve a spray quality of required SMD the current liquid fuel injection systems demand extremely high atomizing gas pressures so that liquid fuel is atomized to the required level. One such technique is being utilized in the design of an effervescent atomizer, which works on the principle stated above.
In effervescent atomizers, first introduced in late 1980s, volatile liquid fuels are atomized into a finely divided spray of small droplets due to injection of high pressurized gas upstream of the discharge orifice. Much finer spray patterns are produced by such atomizers than compared with other conventional atomizers. The atomization of fuel to produce homogenous mixture is important for further combustion process efficiency. The required efficiency of the combustion process in engines of the present day has assumed greater importance than in the past due to increased environmental concerns and more stringent regulations on exhaust pollutants due to inefficient combustion reactions.
The advantages of effervescent atomizers as compared to conventional external or internal atomizers are listed for consideration: (a) Effervescent atomizers have the ability to produce relatively good atomization at much lower gas injection pressures as compared to other atomizers, (b) In comparison to other methods of atomization, much smaller drop sizes are obtained for a given gas injection pressure, (c) The atomizing gas is injected into the liquid at relatively lower velocity to form a bubbly mixture than those employed in most other forms of atomization, (d) For a fixed flow rate, effervescent atomizer gives the luxury of having a larger exit orifice diameter as compared to other atomizers. This prevents orifice erosion in case of liquids having solid suspension. Also this advantage facilitates fabrication and manufacturing processes, (e) Spray characteristics are independent of the fluid properties used. SMD, which is the main performance criteria, is the function of internal design and geometry, (f) Effervescent atomizer has a very durable and simple design. It requires nearly no maintenance and can be operated at low cost
The gas supply pressure has to be kept at slightly higher pressure than the fluid for injection. It is because of the presence of this pressure difference, the atomizing gas is able to flow through the perforations of the central tube and forms a bubbly flow in the liquid stream. Due to this, a two phase homogenous bubbly mixture is formed just upstream of the discharge orifice. The requirement demands presence of a gas pressurization system (probably a centrifugal pump) for a constant supply to the atomizer. Such a pressurized gas system requires considerable space in any of its application.
Thus, there exists a need to re-evaluate the existing effervescent atomizer design in a way that it utilizes all the advantages of a twin fluid internal mixed atomizer, to produce the desired degree of atomization, and at the same time overcome its inherent shortcoming of operating at higher injection pressure of atomizing gas as compared to the liquid fuel.